Method of digesting cellulosic material in steam phase



Oct. 6, 1970 J. c. F. c. RICHTER METHOD OF DIGESTING CELLULOSIC MATERIAL IN STEAM PHASE Filed Sent. 11, 1967 INVENTOR QJM a ATTORNEY5 United States Patent 3,532,594 METHOD OF DIGESTING CELLULOSIC MATERIAL IN STEAM PHASE Johan C. F. C. Richter, St. Jean Cap Ferrat, France, as-

signor to Aktiebolaget Kamyr, Karlstad, Sweden, a company of Sweden Filed Sept. 11, 1967, Ser. No. 666,737

Claims priority, application Sweden, Sept. 12, 1966,

12,221/66 Int. Cl. D21c 3/26 U.S. Cl. 162-19 6 Claims ABSTRACT OF THE DISCLOSURE The present invention concerns a method of continuous digestion of cellulosic fiber material, such as wood chips. The chips are first impregnated with digesting liquor in an impregnation vessel under a reduced pressure and then transferred to a digester, in the upper part of which the fiber-liberating digestion takes place in gas phase. The chips are heated and the digester is held under a high pressure by means of high-pressure steam supplied to the top of digester. Only a part of the liquor quantity required for the digestion of the chips is absorbed by the chips while immersed in digesting liquor in the impregnation vessel. The remainder of the liquor is supplied as a spray or mist of minute drops to the upper end of the digester where it will settle upon the surfaces of the chips particles together with the steam ,thus maintaining proper liquor strength in the surface layers and preventing leaching by the steam condensate. The steam used for heating the digester also serves for atomizing the liquor supplied to the digester. The lower part of digester is filled with liquid driven in counter-current flow to chips for washing out the black liquor thereof.

The present invention relates to the digestion in steam phase of finely comminuted fiber material, such as wood chips, which has been impregnated in advance with digesting liquor by immersion of the fiber material into digesting liquor in a separate impregnation vessel from which the fiber material is transferred, after draining of digesting liquor not absorbed, to the steam space of a digester in which fiber-liberating digestion is performed.

In this method it is difficult to have the chips absorb a sufficiently great quantity of the digesting chemicals, such as alkali, required for the digestion. The reason therefor is that the chips have a high moisture content in their raw condition and that often the moisture content is further increased by their steaming, being a usual preparatory measure to the digestion. Furthermore, the used digesting liquor usually is of a quite moderate strength, this applying particularly to the case when the liquor is taken from a recovery plant where the causticising reaction of the liquor limits its concentration. Therefore, it has been necessary to take measures for removing water from the chips or the liquor, such as drying of the chips, removal of steam by flashing in the impregnation vessel or preevaporation of the digesting liquor.

The invention has for its object to eliminate said difficulties, so that the measures just mentioned become wholly or partly superfluous. According to the main feature of the invention said object is reached thereby that a portion of the total liquor quantity required for the digesting reaction is supplied separately to the steam space of the digester as a spray of minute drops. Then it can be accepted that the impregnation of the chips with digesting liquor performed in the impregnation vessel is incomplete, i.e. after the chips have been raised out of the digesting liquor in the impregnation vessel and loosely adherent liquor has had a chance to drain off, the liquor quantity absorbed by the chips and accompanying the same into the digester, is insufiicient for the digestion, because by the addition of an extra quantity of liquor to the steam space of the digester said deficiency is compensated so that the quantity of liquor required for a complete dissolution of the lignin from the fiber material is present in the digester.

The liquor portion separately supplied to the digester is atomized and distributed in the digester preferably by means of such high-pressure steam as is supplied to the steam space of the digester for the purpose of heating the fiber material to digesting temperature.

Further and secondary features of the invention and preferred embodiments thereof will be evident from the following description wherein reference is had to the accompanying drawing which diagrammatically shows a cellulose digestion plant suitable for carrying out the method according to the invention.

In the drawing, 11 designates an upright cylindrical steaming and low-pressure impregnation vessel, to the upper end of which the finely comminuted fiber material, e.g. wood chips, that should be digested, is supplied via a rotary valve 13 with a pocketed rotor serving as a pressure lock. Steaming of the chips is performed in the upper part of said vessel by means of low-pressure steam supplied through the conduit 15 in a quantity controlled by the valve 17, which steam is spread from the lower end of a vertical tube 19 suspended in the vessel 11. Such air as is displaced by the steam is discharged through the conduit 12 connected to the top of the digester.

The steam condenses upon the chips and heats the same to approximately C. The pressure in the vessel 11 may be of the order of 1 to 2 atmospheres overpressure.

In the lower part of the vessel 11 the chips are impregnated with digesting liquor, such as sulphate liquor, which is supplied through the conduit 21 and is driven by the pump 23 through a heat exchanger 25 and into the vessel through the conduit 27 suspended therein. To the inlet side of the pump there is also supplied diluting liquor, preferably black liquor, which is more or less chemically inactive and which is taken from the conduit 29. The quantity thereof is controlled by a valve 31 controlled by a level control device by means of which the surface of the liquid filling of the vessel can be maintained at a desired level 33, preferably being adjustable. Also connected to the inlet side of the pump 23 is a conduit leading from a screen girdle 35 inserted in the shell of the vessel, the pump thus maintaining a circulation of digesting liquor in a circuit through 35, 23, 25, 27. Said circulating liquor as well as newly supplied white liquor from the conduit 21 and possibly black liquor from the conduit 29 are heated indirectly in the heat exchanger 25 by means of low-pressure steam taken from the conduit 15. The valve 37 in the steam conduit controls the temperature of the circulating liquor, so that the same is maintained at a desired value, e.g., C. Due to said indirect heating of the digesting liquor the steam quantity required for direct heating and supplied through the conduit 19 is decreased. Thus it is possible to reduce and within certain limits to control the dilution of the digesting liquor with steam condensate. If desired, the steam supplied through the conduit 19 may be shut off and the indirect heating made so great that water in the vessel 11 is evaporated, the steam formed departing through the conduit 12.

A suspension of the steamed and impregnated fiber material in digesting liquor is discharged at the bottom of the vessel 11 'by means of a rotary discharging device 39 preferably being of the kind shown in the U.S. Pat. No.

2,960,161. The motor driving said discharging device is controlled by a level control means 41, whereby the discharge takes place at such a rate that the level of the chips at the upper end of the vessel is maintained constant.

The mixture of chips and digesting liquor is pumped to the top of a digester 43 by means of a circulation system held under the digesting pressure and which includes a high-pressure valve 45 and a screening device 47 in open communication with the digester top. The valve 54 comprises a rotary plug having a pocket which, in the shown vertical position, is filled with chips with the aid of liquor circulated by means of the pump 49 in a closed loop on the low-pressure side of the valve. By turning the plug of the valve through an angle of 90 its pocket is inserted into a high-pressure circulation loop consisting of the conduits 51, 53 and the screening device 49, and the content of said pocket will be pumped through the conduit 51 to the screening device 47 by means of the pump 55. Such liquor as penetrates the cylindrical screen 57, is returned through the conduit 53, whereas the chips retained by said screen are fed by means of a motor-driven conveyor screw 59 through the inclined housing of the screening device up to its uppermost end where the chips drop into the digester 43 due to the gravity. A predetermined liquor level 61 is maintained in the screening device by means of a valve 63 controlled by impulses derived from said level, and a pump 65 which returns to the high-pressure circulation loop such liquid as, due to the operation of the valve 45, has been let out into low-pressure circulation loop.

In the upper part of the digester 43 the chips are digested in the gas phase after having been heated by highpressure steam supplied through the conduit 67 in such a quantity controlled by the valve 69 that a pressure of the order of 10 atmospheres and a corresponding temperature, e.g. approximately 170 C., are maintained in the digester. In the lower part of the digester 43 the digested fiber material is washed by means of wash liquid supplied through the conduit 71 and pumped by means of the pump 73 into the lower end of the digester in such a quantity controlled by the valve 75 that the digester is maintained filled with liquid to the desired level 77. The wash liquid is heated indirectly with steam in a loop comprising screen girdle 79, pump 81, heat exchanger 83 and the central tube 85. The wash liquid thus heated is driven upwardly and in counter-current flow to the slowly descending chips column, displacing its content of spent liquor which departs through the screen girdle 87 and is carried off to the blow tank 89. Therein the pressure of the liquor is lowered, steam being released which may be supplied through the conduit 91 to the vessel 11 for the purpose of steaming the chips. In such a case the quantity of steam supplied through the conduit 15 can be lowered in a corresponding degree or may possibly be wholly shut off by means of the valve 17. A forced horizontal and radial flow of liquid is maintained at the level of the screen girdle 87 by means of the pump 93 and the central conduit 95, thereby improving the washing effect. The departing total quantity of liquid and thus the quantity of wash liquid mixed into the spent liquor can be controlled by the adjustable valve 97. The digested fiber material is discharged at the lower end of the digester by means of a rotary scraper 99 and a throttling device 101. By means of a valve 103 inserted in the pulp discharge conduit and a level sensing device 105 the discharge is controlled in such a manner that the digester is maintained filled with chips to the desired level.

According to the invention the digesting liquor is divided up into two portions, the main portion of which being introduced in the above-described manner by means of the pump 23 into the impregnation vessel 11, whereas the remaining smaller portion is supplied by means of the pump 107 and the conduit 109 to the steam space of the digester. Said latter liquor portion is preferably introduced into the high-pressure steam supplied to the upper end of the digester for the purpose of heating the chips to digesting temperature, the steam being utilized to atomize the liquor. Thus the liquor from the conduit 109 may be supplied to an ejector or spreader device 111 inserted in the steam conduit 67, so that the liquor is introduced into the digester in the shape of minute drops together with the steam. When the steam condenses upon the particles of chips, a corresponding quantity of the liquor mist is deposited upon them. The steam is spread in even distribution over the chips charge, because it tends to condense in the first place upon the comparatively cooler particles, and therefore the liquor will also be spread and distributed evenly in the mass of chips. The greater part of the liquor is deposited upon the chips particles already in the outlet of the screening device 47 and during the free gravity drop of the chips through the digester top down to the predetermined chips level. The liquor quantity thus supplied directly to the digester is controlled by means of the valve 113 in the conduit 109, which is operated under control of a quantity measuring and control means 115. The latter is adjusted according to the conditions of operation in order to maintain a predetermined flow in the conduit 109. The total quantity of the supplied white liquor is controlled by a valve 117 in the conduit 21, which is adjusted so as to admit white liquor in a quantity which corresponds to the quantity of chips fed into the vessel 11 by the valve 13 and which is suitable for a proper digestion of the fiber material in the digester. The difference between the white liquor quantities admitted by the valves 117 and 113 goes to the impregnation vessel 11 wherein it is absorbed by the chips. The chips lifted by the feed screw 59 out of the liquid present in the screening device 47 and forwarded to the digester, contain said differential white liquor portion and usually also a certain amount of black liquor and steam condensate as well as water in the shape of the native moisture content of the chips.

The course of absorption in the impregnation vessel may be modified in various manners in order to bring about that the chips carry with them exactly the white liquor portion remaining when the quantity supplied directly to the digester has been deducted from the totally required white liquor quantity. Thus the duration of impregnation may be varied by raising or lowering the liquor level 33. Furthermore, the digesting liquor in the impregnation vessel may be diluted, e.g. by replacing at least part of the black liquor supplied through conduit 29, by water. However, it is usually desired to maintain a high concentration of the liquor and to said end the dilution is reduced in a controlled degree by replacing, wholly or partly, the direct heating with steam from conduit 15 and/or 91, by indirect heating in the heat exchanger 25 or by evaporating in a controlled manner a portion of the liquid content of the chips and the digesting liquor in the impregnation vessel.

In addition to the advantage consisting in that the supply of a portion of the white liquor directly to the digester involves that it is no longer needed to impregnate the chips with the whole quantity of chemicals required for the digestion, which in its turn involves that a higher percentage of moisture of the chips and/or a lower concentration of the white liquor than else can be accepted, the addition of liquor to the steam used for heating the chips in the digester also has the favorable eifect that the condensate formed by said steam and deposited upon the surfaces of the chips, will not dilute the liquor whereby the surface layers of the chips have been impregnated in the vessel 11, but the liquor deposited upon the surfaces of the chips together with the steam serves to maintain approximately the same liquor strength therein as in the interior of the chips, thus favoring an even digestion of the fiber material.

What is claimed is:

1. The method of fiber-liberating digestion of comminuted fibrous material which comprises the steps of impregnating said fibrous material by immersing it in digesting liquor in an impregnation vessel, wherein said fibrous material absorbs a portion of the digesting liquor, draining from the impregnated fibrous material the digesting liquor not therein absorbed, transferring the drained fibrous material to the steam space of a digester, and supplying to said steam space of the digester steam and as an atomized spray or mist of minute drops, the remaining portion of the digesting liquor required to maintain sufficient liquor strength in the surface layers of said comminuted fibrous material to complete dissolution of lignin therefrom and to prevent leaching of the digesting liquor by the steam condensate.

2. Method according to claim 1 which further includes the steps of supplying said remaining portion of the digesting liquor to the steam space by injection with high pressure steam and atomizing said remaining portion of the digesting liquor by means of high-pressure steam supplied to the steam space of the digester for heating the fiber material to digesting temperature.

3. Method according to claim 1 which further includes the steps of supplying said remaining portion of the digesting liquor to the steam space by injection with high pressure steam, and distributing said remaining portion of the atomized digesting liquor upon the fiber material particles during their transfer by gravity drop into the digester.

4. Method according to claim 1 which further includes the step of controlling the time of impregnation by varying the liquor level in the impregnation vessel.

5. Method according to claim 1 which further includes the step of controlling the evaporation of the liquid content of the digesting liquor in the impregnation vessel.

6. The method of claim 1 wherein said comminuted fibrous material is wood chips.

References Cited UNITED STATES PATENTS 2,492,047 12/1949 KBurg et al. 162-17 2,858,213 10/1958 Durant et al. 16217 3,258,390 6/1966 Tomlinson 16217 S. LEON BASHORE, Primary Examiner T. G. SCAVONE, Assistant Examiner US. Cl. X.R. 162-46, 61, 63 

